1. Field of the Invention
The present invention relates to electrical connectors for connecting daughter boards to mother boards and, especially, to DIMM type electrical connectors.
2. Description of the Related Art
An electrical connector for connecting a daughter board to a mother board includes an insulation housing to be mounted on the mother board and a plurality of contact terminals. The insulation housing includes a base and a pair of lock mechanism extending laterally from opposite ends of the base. A slot is provided in the base to receive a front edge of the daughter board and the contact terminals. The contact terminals are soldered to circuit traces of the mother board to secure the insulation housing. The daughter board is fitted into the insulation housing so that the front edge is supported by the slot and that the side edges are securely held by the lock mechanisms. Consequently, the pads of the daughter board are brought into contact with the contact terminals in the slot to connect the daughter board to the mother board.
The known types of electrical connector include dual-in-line memory module (DIMM) and single-in-line memory module (SIMM). In the DIMM type, the pads provided on the front and back sides of a daughter board are brought into contact with the contact terminals which are electrically separated. In the SIMM type, the pads provided on one side or both sides of a daughter board are brought into contact with the same contact terminal.
Examples of the electrical connectors are disclosed in Japanese patent application Kokai Nos. 288153/95 and 211370/95. In these examples, the lock mechanisms extend at right angles with the mother board. A daughter board is inserted from above along the grooves of the lock mechanisms. Projections are provided in the grooves to make them narrower for holding the daughter board between them. A plurality of contact terminals are provided in the base of an insulation housing to hold the front edge of the daughter board between them.
As the number of contact terminals increases, the insertion or removal force of a daughter board increases. Japanese patent application Kokai No. 193473/88 discloses a rotation-lock type electrical connector. In this electrical connector, the front edge of a daughter board is inserted obliquely into the slot of an insulation housing and then rotated rearwardly to a lock position to a mother board. This rotation-lock mechanism minimizes the insertion or removal force. A pair of claws are provided on the lock mechanisms to hook the daughter board, thereby minimizing the coming off of the daughter board.
However, in the rotation-lock mechanisms, the vertical distance between contact terminals on the front and back sides of a daughter board is so large that a large moment of rotation is applied to the daughter board, making the daughter board warped. The contact between the daughter board and the contact terminals is unstable, especially, for a large number of contact terminals or a large width of daughter boards.
The inventors have proposed a solution to the problem of the large vertical distance for the SIMM type connectors, but the problem with the DIMM type connector remains.